Conventional electronic circuits for RF and telecommunications applications comprise one or more input ports to which input RF signals of the electronic circuit are fed, and one or more output ports from which output RF signals of the electronic circuit are emitted. Single-ended input/output ports have a pair of connection terminals: a signal terminal and a ground terminal, where the input and output RF signals of the electronic circuit are carried on the signal terminal and where the ground terminal provides a reference against which the RF signal on the signal terminal is defined.
In RF and telecommunications applications it is sometimes preferable to employ electronic circuits where the input/output (hereinafter referred to as I/O) ports of the device comprise a pair of signal carrying terminals where each terminal carries part of an input or output electrical signal of the electronic circuit.
The pair of RF signals carried on each terminal described above can be individually referenced to ground, or can be described mathematically as a linear combination of two signals: a differential mode signal and a common mode signal. A differential mode signal is divided between two terminals so that the amplitude of the signal on each terminal is the same, and so that there is a phase difference of 180° between both signals; thus the two parts of a differential signal carried on a pair of terminals are out of phase. A common mode signal is divided across two terminals so that the amplitude of the signal on each terminal is the same, and so that both signals are in phase; thus the two parts of a common mode signal carried on a pair of terminals are identical.
RF circuits comprising a pair of signal carrying terminals for each I/O port of the circuit are usually designed to process differential signals and are usually referred to as differential circuits. Sometimes RF circuits comprising a pair of signal carrying terminals for each I/O port of the circuit are referred to as “balanced circuits”.
Differential mode signals are less susceptible to noise than common mode signals and consequently circuits designed to accept differential mode signals are often preferred for applications where a very high signal to noise ration is required. However, it is sometimes more practical to realize a particular device in a single-ended topology (for example single-ended antennae are often preferred to balanced antennae). A device which can convert a single ended signal to a differential mode signal is referred to as a balun.
The simplest type of balun is the half-wave balun. FIG. 1 shows a prior art half-wave balun 10, comprising a single-ended I/O port P1, and a differential I/O port P2. The balun has an operating band characterized by a lower frequency limit FL and an upper frequency limit FU. I/O port P1 comprises a signal carrying terminal T1, and I/O port P2 comprises a pair of signal carrying terminals T2 and T3. Signal carrying terminal T1 is connected to a circuit node 13, which is also connected to signal carrying terminal T2, and which is connected to signal carrying terminal T3 via a length of transmission line 14 with an electrical length E of 180° at the centre frequency of the operating band of the balun.
An RF signal which is incident on terminal T1 is divided into two parts with the same amplitude at circuit node 13, one part of the RF signal is fed directly to terminal T2 and another part of the RF signal is fed to terminal T3 via transmission line 14 so that the RF signals which are emitted at terminals T2 and T3 will have the same amplitude, and will have a phase difference of 180° at the centre of the operating band of the balun. Thus, it is apparent that the half-wave balun of FIG. 1 has the required properties, i.e. a single ended signal incident at I/O port P1 will be emitted as a differential mode signal from I/O port P2 and a differential mode signal incident at I/O port P2 will be emitted as a single ended signal from I/O port P1.
The half-wave balun of FIG. 1 has the drawback of being very large at the operating frequencies of typical commercial cellular and W-LAN applications. For example, at an operating frequency of 2.45 GHz, the centre of the band specified in IEEE 802.11b/g for W-LAN applications, a half wavelength transmission line will have a length of 61.22 mm in air and will have an electrical length given by the expression below for a transmission line fabricated in a dielectric material.
                    λ        2                          f      =              2.45        ⁢                                  ⁢        GHz              =            61.22                        ɛ          r                      ⁢                  ⁢    mm                  where ∈r is the relative dielectric constant of the material.        
Other balun designs have been proposed for applications requiring a compact solution.
FIG. 2 shows a Marchand balun with capacitive loading at the input and output terminals such as that disclosed in “A semi-lumped balun fabricated by low temperature co-fired ceramic”; Ching-Wen Tang, Chi-Yang Chang; 2002 IEEE MTT Symposium Digest, Volume: 3, pp: 2201-2204. A similar balun is disclosed in U.S. Pat. No. 6,483,415, “Multi-layer LC resonance balun”, Tang. The Marchand balun 20 of FIG. 2 comprises a first pair of coupled transmission line sections 23A and 23B and a second pair of coupled transmission line sections 24A, 24B where each of transmission line sections 23A, 23B and 24A, 24B has substantially the same electrical length and where the even mode and odd mode impedances of first pair of coupled transmission line sections 23A and 23B are substantially the same as the even mode and odd mode impedances of second pair of coupled transmission line sections 24A and 24B. The Marchand balun 20 of FIG. 2 further comprises a single-ended I/O port P1 comprising a signal carrying terminal T1 connected to an end of coupled transmission line section 23A, and differential I/O port P2 comprising a pair of signal carrying terminals T2 and T3 connected to ends of coupled transmission line sections 23B and 24B as shown in FIG. 2. Loading capacitors 26, 27, 28 and 29 are also connected to ends of coupled transmission line sections 23A, 23B and 24A, 24B as shown in FIG. 2. The effect of loading capacitors 26, 27, 28 and 29 being to allow the use of coupled transmission line sections which have an electrical length E which is less than 90° at the centre of the operating band of the balun 20.
FIG. 3 shows an LC balun according to FIG. 1C of U.S. Pat. No. 5,949,299: “Multilayered balance-to-unbalance signal transformer”, Harada. The LC balun 30 of FIG. 3 comprises inductor 34, capacitor 35, inductor 36 and capacitor 37 connected together at circuit nodes 33A, 33B and 33C as shown in FIG. 3. The LC balun 30 of FIG. 3 further comprises a single-ended I/O port P1 comprising a signal carrying terminal T1 connected to a first circuit node 33A, and differential I/O port P2 comprising a pair of signal carrying terminals T2 and T3 connected to second and third circuit nodes 33B and 33C respectively.
The LC balun 30 of FIG. 3 can be realized in a compact form, for example using a multilayer low temperature co-fired ceramic (LTCC) structure as described in Harada.
A procedure for the analysis of electronic circuits or devices comprising one or more differential I/O ports is outlined by D. E. Brockelman, W. R. Eisenstadt; “Combined Differential and Common-Mode Scattering Parameters: Theory and Simulation”; IEEE Transactions on Microwave Theory and Techniques, Vol. 43, No. 7, July 1995, pp 1530-1539. For a device with a single-ended I/O port and a differential I/O port the relevant parameters are:
SDS21, the differential mode response at the differential port for a stimulus at the single-ended port;
SCS21, the common mode response at the differential port for a stimulus at the single-ended port;
SDD22, the differential mode reflection coefficient at the differential port for a differential mode stimulus at the differential port;
SCC22, the common mode reflection coefficient at the differential port for a common mode stimulus at the differential port;
SSS11, the single-ended reflection coefficient at the single ended port.
FIG. 4A shows typical through responses of the LC balun 30 of FIG. 3 where inductors 34 and 36 both have inductances of 0.65 nH, and where capacitors 35 and 37 both have capacitances of 6.5 pF. The balun is designed to convert a single ended signal to a differential mode signal within a passband from 2400 MHz to 2500 MHz in line with the IEEE 802.11b/g standard for W-LAN applications. It can be seen that the differential mode response of the LC balun 30 of FIG. 3 is excellent (offering very low insertion loss within the passband). The maximum value of the common mode response within the passband is −33 dB approx; this is an acceptable level, though ideally, for a balun, the common mode response would be lower.
FIG. 4B shows the through responses of the LC balun 30 of FIG. 3 over a wide frequency range and with the same parameters as FIG. 4A. It can be seen that the common mode response of the LC balun 30 of FIG. 3 increases monotonically with increasing frequency above the passband and increases monotonically with decreasing frequency below the passband. Consequently, the balun of FIG. 3 is unsuitable for applications where a high common mode signal level far outside the passband of the balun gives rise to problems in the circuitry to which the balun is connected.
Another drawback of the LC balun 30 of FIG. 3 is that it requires two inductors 34 and 36. Unfortunately, if the circuit is to be fabricated using LTCC materials with a high dielectric constant, the realization of high Q inductors is difficult, and the insertion loss of the circuit becomes high.
For example, multilayer LTCC substrates with a layer thickness of 40 μm and a dielectric constant of 75 are typical for RF applications at 2.45 GHz. The resulting capacitance between mutual windings of an inductor is sufficiently large to lower the self resonant frequency of the inductor to a frequency below 2.45 GHz.
A further drawback of the LC balun 30 of FIG. 3 is that a pair of bias-tee networks are required in order to apply a DC bias to signal carrying terminals T2 and T3 of I/O port P2.